Japanese Laid-Open Patent Publication No. 2000-15586 discloses an example of a conventional electric power tool. The electric power tool of the publication (refer to FIGS. 1 and 6 of the publication) includes a gear case (8), a motor, a ring gear (23), a planet gear (29), a carrier (30), an output shaft (9), a torque clutch, a clutch sensor (S), and a shutoff circuit. The torque clutch includes a ring gear protrusion (32), a ball (33), a clutch plate (36), and a clutch spring (37).
The motor rotates the planet gear. The planet gear engages with the ring gear. A pin of the carrier is inserted into the planet gear. The carrier is coupled to the output shaft and rotated integrally with the output shaft. The clutch spring applies a load to the ring gear via the clutch plate and the ball.
When a load torque acting on the output shaft is less than a predetermined torque, the rotation of the ring gear with respect to the gear case is restricted by the load input from the clutch spring. When the load torque acting on the output shaft is greater than or equal to the predetermined torque, the ring gear is rotate relative to a housing by a torque input from the planet gear. Thus, when the load torque is greater than or equal to the predetermined torque, the planet gear does not input torque to the carrier. Accordingly, the torque of the motor is not transmitted to the output shaft. When the load torque is greater than or equal to the predetermined torque, the torque clutch disconnects a torque transmission line between the motor and the output shaft.
When the ring gear rotates relative to the gear case, the ball is moved by the protrusion of the ring gear. This moves the clutch plate. The clutch sensor outputs a signal when the clutch plate moves. The shutoff circuit outputs a motor stop signal when the signal of the clutch sensor rises. The motor stops rotation in response to the motor stop signal.